Phosphorous and maleic acid buffers for ferricyanide photographic bleaches



Sept. 19, 1967 3,342,598

C. C. BARD PHOSPHOROUS AND MALEIC ACID BUFFERS FOR FERRICYANIDE PHOTOGRAPHIC BLEACHES Filed June 14, 1965 BUFFER CAPAC/ TY CHARLE TON C BARD I N VEN TOR.

ATTORNEY AND AGENT United States Patent Cilice Patented Sept. 19, 1967 3,342,598 PHOSPHOROUS AND MALEIC ACID BUF- FERS FOR FERRICYANIDE PHOTO- GRAPHIC BLEACHES Charleton C. Bard, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey Filed June 14, 1965, Ser. No. 463,698 19 Claims. (Cl. 96-60) This invention relates to photography and more par ticularly to bleach baths used in silver halide photographic processes.

It is known that in the processing of multicolor photographic film and papers, that the silver formed in all the layers during development is converted into a silver salt by a bleach solution which contains an oxidizing agent, such a an alkali metal ferricyanide, a buffer and in most instances an alkali metal halide, such as sodium chloride or potassium bromide. The silver salt formed by action of the bleach on the silver is then removed by dissolving it in a silver-salt solvent, such as a conventional alkali metal thiosulfate bath.

It is known that pH control in bleach baths is very important. For example, if the pH is too high, silver retention may occur, or if the pH rises above 7, stains are likely to be produced in the photographic material, or if the pH falls below 6 the stain again increases. While many buffers have been used in bleaches, such as ferricyanide bleaches, including such materials as borax, phosphates and carbonates, good buffers have never been available in the pH range of from 6 to 7. Particularly troublesome is the tendency for ferricyanide bleaches to form Prussian blue at pHs below about 6.

It is, therefore, an object of my invention to provide a novel class of buffers that are stable in photographic bleach solutions and that will buffer the solutions in a pH range of from about 6 to about 7, and which have an increased buifering capacity as the pH approaches 6 from a pH of 7.

Another object is to provide novel bleach compositions for preparing bleach solutions which are stable and which are strongly buffered in the pH range from about 6 to about 7, such that the buifering capacity of the said solution increases as the pH approaches 6.

Still another object is to provide a bleach step for a color photographic process utilizing my novel bleach solution, said bleach causing no dye or Prussian blue formation in the photographic element being processed.

Still other objects will be apparent from a consideration of the following specification and claims.

These and other objects are accomplished according to my invention by using as the buffer in my bleach compositions and bleach baths, phosphorous acid, a watersoluble salt of phosphorous acid, maleic acid or a watersoluble salt of maleic acid. The water-soluble salts of phosphorous acid used to advantage according to my invention include the alkali metal phosphites (e.g., sodium phosphite, potassium phosphite, lithium phosphite, etc.), ammonium phosphite, an alkali metal ortho phosphite (e.g., sodium ortho phosphite, potassium ortho phosphite, lithium ortho phosphite, etc.), ammonium ortho phosphite, an alkali metal acid phosphite (e.g., sodium acid phosphite, potassium acid phosphite, lithium acid phosphite, etc.), ammonium acid phosphite, etc. The Watersoluble salts of maleic acid include the alkali metal salts (e.g., monosodium maleate, disodium maleate, monopotassium maleate, dipotassium maleate, monolithium maleate, dilithium maleate, etc.), the monoand diammonium maleates, etc. My bulfered bleach compositions and solutions are used advantageously for preparing a combined bleach-fix bath by adding a suitable silver salt complexing agent.

My bleach composition contains as the oxidizing agent a water-soluble ferricyanide, such as an alkali metal ferricyanide (e.g., potassium ferricyanide, sodium ferricyanide, lithium ferricyanide, etc.), ammonium ferricyanide, etc.

I have found that my buffers can be used to advantage over a wide range of concentrations depending upon the particular processing conditions, photographic materials, etc. For example, I have found that concentrations of buffer up to about g./l. are useful; however, usually a concentration up to 30 g./l. is preferred. Usually the oxidizing agent is used in the range of from about 5 g./l. to about 200 -g./l.

Any of the known silver salt complexing agents can be used to advantage in my bleach compositions to make a blix composition. Included among the useful complexing agents are the water-soluble thiocyanates, such as, the alkali metal thiocyanates, ferric thiocyanate, etc. dithiosuberic acid, etc. The alkali metal thiosulfates can be used; however, they are not prefer-red for use in a ferricyanide bleach because of the short life of Farmers reducer formed. The complexing agents can be used over a wide range of concentrations. For example, the watersoluble thiocyanates can be used to advantage at concentrations from about 40 to about grams per liter. Dithiosuberic acid can be used to advantage at concentrations from about 20 to about 150 grams per liter. These materials can be preweighed and added dry to the dry bleach compositions. Sufficient dry material is included to produce a solution having the desired concentrations.

My bleach solution can contain any of the various addenda that are usually used in bleach solutions, including alkali metal halides (e.g., potassium bromide, sodium chloride, etc.), bleaching accelerators, such as hexyl Cellosolve, etc., potassium ferrocyanide, potassium persulfate, alkali metal nitrates, boric anhydride, borax, citric acid, monosodium phosphate, phosphoric acid, alkali metal hexametaphosphate, etc.

It is known that phosphoric acid has a decreasing buffering capacity as the pH is lowered from 7 and that because of this bleaches using the ferricyanide ion as the oxidizing agent will and do form Prussian blue when relatively small amounts of acid are added. The literature suggests that phosphorus acid is oxidized to phosphoric acid by ferricyanide ion at pHs below 7. It is, therefore, not obvious that phosphorous acid would be stable (i.e., not be oxidized to phosphoric acid) in the immediate bleach solutions and that the immediate bleaches containing phosphorous acid and ferricyanide ion would not form Prussian blue.

It is not obvious that maleic acid or its water-soluble salts would be stable in the immediate bleach solutions and that these compounds would be valuable buffers in my bleach compositions.

I have found that both phosphorous and maleic acids and their water-soluble salts are not only stable in the immediate bleach solutions but are stable in bleaches outside the invention using quinone as the oxidizing agent for example, or even using the dichromate ion as a bleach in the pH range of from 1 to 3. In the latter, extreme conditions, the phosphorous acid has been found to be stable for several days and the maleic acid has been found to be stable for about a day. Both the phosphorous and maleic acids and their water-soluble salts can be used to advantage to buffer alkali metal dichromate solutions in the pH range from about 6 to about 7, however, the dichromate ion is not a good oxidizing agent for silver at pHs much above'3.

My bleach compositions containing the water-soluble salts of either phosphorous or maleic acids are advantageously prepared and stored in the dry form for solution in water to prepare the bleach bath when needed for processing. My dry bleach compositions are made to contain the necessary amount of material to produce a solution containing from about 5 to about 200 g./l. of one of my oxidizers and up to about 100 g./l. of one of my buffers, and where needed the desired amount of addenda.

The bleach baths employed according to my invention are used to advantage in any photographic process in Which silver images are present in a photographic layer together with dye images. The dye images may be those formed by color development as described in Mannes et al. U.S. Patents 2,252,718, 2,113,329 and 2,304,940 or the Jelley and Vittum US. Patent 2,322,027. These dyes are of the azomethine, indoaniline or indophenol type. The dyes may also be azo dyes such as those used in the silver dye bleach process of color photography. After all the steps of color processing, the photographic layer or element is treated with my bleach bath followed by a conventional hypo fixing bath to remove the silver salts formed by the bleach bath.

The following examples will serve to further illustrate my invention.

Example 1 Conventional multilayer, multicolor reversal film that contained incorporated color-forming couplers and which had been image exposed, processed through conventional formalin prehardener, acetic acid neutralizer, hydroquinone type negative developer, first stop bath, reversal bath, primary aromatic amino color developer, second stop bath was bleached in a solution having the following composition:

g./I. Sodium ferricyanide 85.0 Sodium bromide 38.0 Phosphorous acid 30.0 Water to 1.0 liter. pH 7.0.

then fixed in a conventional sodium thiosulfate fixing bath and treated in a conventional stabilizing bath. The processed film was of good quality and showed no evidence of unbleached silver or of Prussian blue. The bleach bath pH showed good stability during a period of continuous processing with replenishment never dropping below a pH of 6.3.

The example was repeated using a bleach outside my invention having the same formula except that 1 gram per liter of borax was substituted for the phosphorous acid. The bleach bath pH which was about 7.0 as mixed tended to go below 6 during continuous processing and objectionable Prussian blue stain was produced on the processed film.

Example 2 Conventional multilayer, multicolor reversal film that did not contain any incorporated color-forming couplers which had been image exposed and processed through a conventional color process including the steps of prehardening, negative development, selective re-exposure of the bottom red-sensitive layer, cyan color development, selective re-exposure of the top blue-sensitive layer, yellow color development, fogging of the middle greensensitive layer, magenta color development with the usual intermediate water washes, was bleached in my bleach solution buffered with phosphorous acid and described in Example 1, then fixed in a conventional sodium thiosulfate fix bath. The processed film was of good quality and showed no evidence of unbleached silver or Prussian blue stain. The bleach bath pH showed good stability during a period of continuous processing with replenishment.

The example was repeated using a bleach outside my invention having the same formula except that 1 gram per liter of borax was used in place of the phosphorous acid. The pH tended to go below 6 during continuous processing with the formation of objectionable Prussian blue stain on the processed film.

Example 3 A conventional multilayer multicolor reversal paper that contained incorporated color-forming couplers was exposed to a color image then given conventional color processing through a 1st MQ negative developer, 1st acid stop bath, re-exposure, a primary aromatic amino color developer, a hardener stop bath, then bleached in a solution having the formula:

G. Trisodium phosphate (12H O) 45-0 Phosphorous acid 10.0 Sodium hydroxide 3.35 Potassium ferricyanide 40.0 Sodium bromide 13.0

Water to make 1 liter. pH adjusted to 6.9

and finally was given conventional hardening fix and conventional stabilization. The processed paper prints had good quality and were free of Prussian blue stains. The bleach bath pH level was stable during continuous processing never going below a pH of 6.3 indicating good buffering from the phosphorous acid in my bleach.

Paper prints processed as in Example 3 but bleached in a bleach bath containing no phosphorous acid were found to have objectionable Prussian blue stain and the pH dropped to about 4.5.

Example 4 Similar good results were obtained by repeating Example 3 using 10 g./l. of maleic acid in my bleach in place of phosphorous acid. Silver bleaching was complete and there was no Prussian blue stain formed in the processed paper prints. The pH of the bleach bath never Went below 6.3 during continuous processing conditions.

Results similar to those obtained from use of my bleach solutions in Examples 1, 2 and 3 are obtained when up to 100 grams per liter of phosphorous acid or a water soluble salt of phosphorous acid, such as, an alkali metal (or ammonium) salt including, for example, sodium phosphite, potassium ortho phosphite, sodium acid phosphite, etc., is used as the buffer in my bleach. Similarly, I have found that up to 100 grams per liter of maleic acid or a water soluble salt of maleic acid, such as, an alkali metal or ammonium salt are used to advantage as the buffer in my bleach baths.

The alkali metal ferricyanide can be used to advantage over a range of from about 5 to about 200 grams per liter in my bleach baths. I have found that a bleach such as that used in Example 1 containing phosphorous acid can be mixed using in place of the g./l. of sodium ferricyanide about 182 g./l. of sodium ferrocyanide decahydrate and 48 g./1. of potassium persulfate. In this way the required sodium ferricyanide is prepared by oxidation of sodium ferrocyanide with potassium persulfate without oxidizing the phosphorous acid. It is not obvious that this could be done, since one would expect that the persulfate would first oxidize the phosphorous acid (phosphite ion) rather than the ferrocyanide ion since phosphorous acid is a much stronger reducing agent than is the ferrocyanide The bleach baths used in Examples 1, 2 and 3 were prepared by adding water to the dry ingredients which had been weighed out previously and stored as the dry bleach compositions until they were needed for processing the film or paper materials.

Example 5 Blix baths having the typical compositions given below are used to advantage to bleach and fix in one step, conventional color film and color paper which have been given color processing up to the bleach step.

The accompanying drawing still further illustrates my invention. The drawing shows curves A and B which show the butter capacity vs. pH for 0.05 M potassium acid phosphite and for 0.05 M potassium acid phosphate, respectively. The buffer capacity is the number of gram equivalents of a strong acid that are required to decrease the pH one unit in one liter of solution, assuming no volume change.

It can be seen from the drawing curve A that the solution of potassium acid phosphite has a sharply increasing buffer capacity as the pH of the solution is lowered from 7 toward 6. Thus my bleach solution containing ferricyanide ion (oxidizer) and potassium acid phosphite is particularly valuable in resisting the formation of Prussian blue stain.

Curve B shows that the solution of potassium acid phosphate has a sharply decreasing buffer capacity as the pH is lowered below about 6.75, and that a bleach solution containing ferricyanide ion and this material as the buffer would tend to form Prussian blue stain.

My bleach compositions containing phosphorous acid, a water-soluble salt of phosphorous acid, maleic acid or a water-soluble salt of maleic acid as the buffer are valuable for preparing bleach baths for photographic processing of color films and papers.

It is not obvious that my buffers would be stable under the strong oxidizing conditions in the bleach baths. One would expect phosphorous acid (or the phosphite ion) to be oxidized to phosphoric acid (or the phosphate ion) which is a poor buffer since its buffer capacity decreases sharply as the pH is forced below about 6.75. My bleach baths are characterized by being strongly buffered in the 6 to 7 pH range with increasing buffer capacity as the pH is forced toward 6. This characteristic is especially valuable in bleaches containing the ferricyanide ion since my buffer strongly resists the formation of Prussian blue stain.

The invention has been described in detail with particular reference to preferred embodiments thereof but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

I claim:

1. A dry composition for preparing a buffered bleach solution that is strongly buffered in the 6 to 7 pH range with increasing buffer capacity as the pH is forced toward 6, for use in a silver halide photographic process in which a silver image is present, said composition comprising:

(1) a water-soluble ferricyanide and (2) a buffer selected from the class consisting of phosphorous acid, water-soluble salts of phosphorous acid, maleic acid and water-soluble salts of maleic acid.

2. A dry composition for preparing a buffered bleach solution that is strongly buffered in the 6 to 7 pH range With increasing buffer capacity as the pH is forced toward 6, for use in a silver halide photographic process in which a silver image is present, said composition comprising:

(1) sufficient water-soluble ferricyanide to produce a solution containing from about to about 200 grams per liter, and

(2) sufiicient buffer selected from the class consisting of phosphorous acid, water-soluble salts of phosphorous acid, maleic acid and water-soluble salts 6 of maleic acid to produce a solution containing up to about grams per liter.

3. A composition for preparing a buffered bleach solution that is strongly buflered in the 6 to 7 pH range with increasing buffer capacity as the pH is forced to Ward 6, for use in a silver halide photographic process in which a silver image is present, said composition comprising:

(l) sufiicient potassium ferricyanide to produce a solution containing from about 5 to about 200 grams per liter, and

(2) sufiicient phosphorous acid to produce a solution containing up to 100 grams per liter.

4. A composition for preparing a buffered bleach solution that is strongly buffered in the 6 to 7 pH range With increasing buffer capacity as the pH is forced toward 6, for use in a silver halide photographic process in which a silver image is present, said composition comprising:

(1) sufiicient potassium ferricyanide to produce a solution containing from about 5 to about 200 grams per liter, and

(2) sufiicient sodium acid phosphite to produce a solution containing up to 100 grams per liter.

5. A composition for preparing a buffered bleach solution that is strongly buffered in the 6 to 7 pH range with increasing buffer capacity as the pH is forced toward 6, for use in a silver halide photographic process in which a silver image is present, said composition comprising:

(1) suflicient potassium ferricyanide to produce a solution containing from about 5 to about 200 grams per liter, and

(2) sufiicient maleic acid to produce a solution containing up to 100 grams per liter.

6. A dry composition for preparing a buffered blix solution that is strongly buffered in the 6 to 7 pH range with increasing buffer capacity as the pH is forced toward 6, for use in a silver halide photographic process in which a silver image is present, said composition comprising:

(l) a water-soluble ferricyanide,

(2) a water-soluble silver salt complexing agent, and

(3) a buffer selected from the class consisting of phosphorous acid, water-soluble salts of phosphorous acid, maleic acid and Water-soluble salts of maleic acid.

7. A dry composition for preparing a buffered blix solution that is strongly buffered in the 6 to 7 pH range with increasing bufier capacity as the pH is forced toward 6, for usein a silver halide photographic process in which a silver image is present, said composition comprising:

(1) sufficient water-soluble ferricyanide to produce a solution containing from about 5 to about 200 grams per liter,

(2) suificient water-soluble silver salt complexing agent to produce a solution containing from about 20 to about grams per liter, and

(3) sufiicient buffer selected from the class consisting of phosphorous acid, water-soluble salts of phosphorous acid, maleic acid, and water-soluble salts of maleic acid to produce a solution containing up to about 100 grams per liter.

8. A dry composition for preparing a buffered blix solution that is strongly buffered in the 6 to 7 pH range with increasing buffer capacity as the pH is forced toward 6, for use in a silver halide photographic process in which a silver image is present, said composition comprising:

(1) sufiicient potassium ferricyanide to produce a solution containing from about 5 to about 200 grams per liter,

(2) sufficient dithiosuberic acid to produce a solution containing from about 20 to about 150 grams per liter, and

(3) sufiicient phosphorous acid to produce a solution containing up to 100 grams per liter.

1 y I 1 I I 1 a awmmPOSia-im for preparing a buffered bliriso 1 i f present, said solution comprising :40; .;g./l1 of; potassium p I 1 l I I 1 I I I II lution that is strongly buttered: in the 6 to 7 :pH: range I ferricyanideand 3Q g./l. ofmaleic acid. I I v I i I I I p I I I 1 1 1 withinoreasing buffercapacity as thel uisitorcedgt ward 1 '1 'An qu 'bliXt solutionthat is g y b fiered I I 6, torusein a silver halide photographic process inwhich I Y 1 m the: 6 o :7 ir t ra e wi h i i i sin bu fe ca ty I l I g I a silver image 1is p re1 sent; said composition comprising: 5 I as the. pH is gtorcedj toward 6, for use 'in asilver halide I 1 I I s 1 1 (1) sufficient potassium ferrioyanide; toproduce ;a so 1' photographic; process; in which a silver image is present, 1 I 1; 1 I 1 I i I lution: containing from about 51to about: ZOO'gi-ains I 1 1 said solution; Qorfnpris 1 7 f 1 /lg; of potassium I ferri I I i For liter, 1 1 I I I I I cyanide, 9o g./i. or dithios'uberic' acid, and 30 g. /1l;;of 1 1 1 I 1 i 1 Q 11(2) suflicient sodium thiocyanate to produce :a: win 1 1 1 phosphorus acid. .1 l 1 y 1 1 1 1 1 1 I, p p g 1 1 1 1 11 1 1 ti'on containing from, about 401m about ISOgrams o I 17; An' aqueous 5 solution; that is: 1 strongly buttered 5 1 j I 1 j I 1 1 1 pperliteijandi 1i1j j, f I inthe 6 to 7 pHrange withincreasingbuffercapacity LL11}1f 1 1 I 1 1 I 1 s I 1 1 (3) sufficient phosphorous: acid, to, produce a: solution: 1' i i as the pH is forced toward 6,:for use in, a silver halide 1 1 I p I p 1; 1; I i 1 I 1 containing up to 100 grams per liter; 1 s 1 1 1 I I I photographic process in which a silver "image ispresent; 1 j I 5 1 II 1 1 V V 1 s 10; An aqueous-buttered bleach solutionthat is strongly I I I said solution comprising 125 'g l. of 1 potassiurnf fern- I 1 I1 1 -i 1 1 1; i 1

1 1 buffered in the 6 to '7 pH range with increasing butter 15.1 cyanide, ,IOOgL/L,of sodiumthiocyanate, and 3Q g./l. of 1 1 1 I I capacity a'sjthejpH is forced. towardo, for; use; in asilver I phosphorousacid 1 11; I, 1 p I I 1 I I I f i I 5 halide photograph c process, {in which :a: silver image; is j I I 1 j I 1851p a photographic process of silver halide; color l 1 1 I I 1 I 1 I Q presengsaid solution comprising: f 1 j i 1 i t 1 j V I i I 1' I E 1 Photography inwhich a'silver image and a coi or'ima'g e 1 1 I 1 I I I I 1 1 1 i 1 i 1 (1) from about i 5E :01 about 290 gr ms p r 1 iter of a g I are produced I by color-1 development: and, then subse i j I y I I I 1 i p 1 I g I 1 1 1 1 1 I 1 1 1' Water-soluble ferricyanide, and; 1 1 v 1 I 1 I 1 1 I I 1 201 quently the said silver image is converted to a silver I I 1 1 1 1 i i 1 1 s 1 salt image b tbieaching; step and the r the: said silven 1 1 p I f 1 f I p p sa1t1i ag{ 7 I ernoved by: a? rising step, the improvement I I 1 a d 1 g 7 comprising theuse 1n the said bleaching {step oiaibutfe'red 1' I I 1 1 1 1 j bieach solutio that is strongly buffered in e' 6 to 7 i 1 d a h 1 25' 1 11-1; range fWitlf'l: increasing? buffer} capacity; as he pH; is I 1 I 1 Z 1 1 5' 1 i7 :pfl;r g 1 j I f forced toward 6, said solution comprising; 11 1; I I I 1' 1E I 1' -1 1 ced toward I i I (11;) i fr m ab u j a ou .00 grams Pe liter o 1 1 T water-soluble ferricyanide; 1 and 1 1 .1 1 Z 1 i E 1 l 1 j 1 1 1 1 II I i (2) to about 100 grams per liter of ai buffer se- 1 looted firornfthe class consisting of pho phorous acid, 1 water-soluble salts? of phosphorus i iid, a il r d and Watr soluble salts of maleic add; i I I I I I I I I (21) ibIOut 0 s me 'pe'r iiteriet arena-1m 1 v 1 n st n of j pho pho f vvatersoluhl'ei salts; of phosphorous a "d, lrnalei 1 ;('1;)f sufiicient water-{soluble terrijcyanide to produce a I 1 1 1 1 1 1 I 1 I 5 solution con-tainingfrorn jaboutS to about ZOOgrarns I i I I I l l ;i ;perliter,1and 1 111 3111 11 1 (2 sufficient buffer selectedfrom' the class consisting 1 1 9: 1 1 p o r ph s Pr 1 of 16010.: ph p y 7 1 10fma1eicjacid andwater-sjolubie salts of maleicacid n h s v i n a 0 m mase lp fl s 5 p o uc 1aj Qlutipnc mahi zuP'tQ: 1 g 1 by color development and then subsequently thegsaid 1 1 lit 1 :j': silverimage and the:silver saltsareremovedbyahlix'i l I 1 12. A: composition for preparingabuttered biases soaf s a h: m m n com s neth u i a i 1 f lutionthatis: strongly buffered; in the 1 6 tolTpH range j step. of: a; buffered blix solutiori thatis strongly: buttered;

:vvith increa rig burreiica aeityas thepHfisforce tow f i m u? i 1pfli ja sewith: in rca i igi v er p c y I 1 -1 6, for use in a silver halide Photographic Process in wares mo P is forced Toward mposition fl p f figi l 1 I I 1 i i i I g I a silver image is present, said composition comprising: (1) from about 5 to about 200 grams P liter of a (1) suflicient Water-soluble ferricyanide to produce a Wateusolublo fel'ricyanide,

solution containing from about 5 to about 200 grams (2) from about to about 150 grams P liter of a per liter, and Water-soluble silver salt complexing agent, and (2) sufficient buffer selected from the class consisting P to about 100 grams P liter of a buffer of phosphorous acid and water-soluble salts of maleic looted from the class Consisting of Phosphorous acid, acid to produce a Solution Containing up to 100 Water-soluble salts of phosphorous acid, maleic acid grams per liter. and water-soluble salts of maleic acid. 13. An aqueous bleach solution that is strongly buttered in the 6 to 7 pH range with increasing buffer References Cited gapaicity as the pH is forced toward 6, for use in a silver UNI STATES PATENTS ai e photographic process in which a silver image is present, said composition comprising 85 g./l. of sodium g i 9660 ferricyanide and 30 g./l. of phosphorous acid. e Wfir 96 6o 14. An aqueous bleach solution that is strongly OTHER REFERENCES bufferod in the 5 o 7 P range with increasing u The Condensed Chemical Dictionary, New York, Reincapacity as the pH is forced toward 6, for use in a silver hold Publishing Corporation 1961 hallde p f p o ProCess Whloh a sllver Imago 1S The Focal Encyclopedia of Photography, London, The present, said solution comprising 40 g./l. oi": potassium Focal s, 1958, vol. 2, pp. 982-985. ferricyanide and 10 g./l. of phosphorous acid.

15. An aqueous bleach solution that is strongly buffered in the 6 to 7 pH range with increasing buffer NORMAN TORCHIN Primary Exammer' capacity as the pH is forced toward 6, for use in a silver DAVIS Assistant Examinerhalide photographic process in which a silver image is 

18. IN A PHOTOGRAPHIC PROCESS OF SILVER HALIDE COLOR PHOTOGRAPHY IN WHICH A SILVER IMAGE AND A COLOR IMAGE ARE PRODUCED BY COLOR DEVELOPMENT AND THEN SUBSEQUENTLY THE SAID SILVER IMAGE IS CONVERTED TO A SILVER SALT IMAGE BY A BLEACHING STEP AND THEN THE SAID SILVER SALT IMAGE IS REMOVED BY A FIXING STEP, THE IMPROVEMENT COMPRISING THE USE IN THE SAID BLEACHING STEP OF A BUFFERED BLEACH SOLUTION THAT IS STRONGLY BUFFERED IN THE 6 TO 7 PH RANGE WITH INCREASING BUFFER CPAPCITY AS THE PH IS FORCED TOWARD 6, SAID SOLUTION COMPRISING: (1) FROM ABOUT 5 TO ABOUT 200 GRAMS PER LITER OF A WATER-SOLUBLE FERRICYANIDE, AND (2) UP TO ABOUT 100 GRAMS PER LITER OF A BUFFER SELECTED FROM THE CLASS CONSISTING OF PHOSPHOROUS ACID, WATER-SOLUBLE SALTS OF PHOSPHORUS ACID, MALEIC ACID AND WATER-SOLUBLE SALTS OF MALEIC ACID. 